In industrial processes such as electroplating, electropolishing and anodizing, and in many other applications, electrical parameters such as the electrical conductivity and dielectric constant of certain fluids have a substantial impact. In such industries and applications, real-time monitoring of the fluid's electrical parameters can be of significant importance.
Many devices and methods have been developed for measuring the electrical conductivity of fluids. Some methods involve immersing a pair of electrodes in the fluid and measuring the resistance, or alternatively the conductance, between the electrodes. The measured resistance across the electrodes is proportional to the electrical resistivity of the fluid with which they are in contact. The proportionality factor, called the cell constant, is theoretically derivable from the geometry of the electrode configuration, with possible geometries including pairs of parallel plates, side-by-side cylindrical electrodes, and coaxial cylindrical electrodes. The known proportionality factor combined with the measured resistance or conductance yields a determination of the electrical conductivity of the fluid.
Typically, the prior devices measure electrical conductivity by first calibrating the device with a standard fluid having a known electrical conductivity. By immersing the device in the standard fluid and measuring the resistance across the electrodes, the cell constant of the device is quantified. Thereafter, resistivity measurements can be made on other fluids by immersing the device of presumed known cell constant into the fluid and measuring the resistance across the electrodes. Since the resistance and proportionality factor (cell constant) are known, the electrical conductivity of the fluid is obtained.
This prior approach relies on the premise that the relationship between the electrical conductivity of the fluid and the resistance measured by the device is completely attributable to the geometry of the device, which is presumed not to change and is independent of the fluid in which the device is immersed and the operating conditions of the device. However, it is known that certain fluid-dependent effects, of which this prior calibration procedure does not take account, attend these resistance measurements. Among the effects are fringe current paths which appear electrically in parallel with the normal current paths between the electrodes. These parallel or shunt current paths appear as parallel conductances across the electrodes which, depending upon the fluid, its container and the container surfaces, can have a marked effect on the resistance measurement obtained. That is, since these fringing effects can vary substantially with fluid and measuring conditions, the typical prior calibration-based procedure can have substantial inherent inaccuracies.
In other prior systems, in an effort to precisely control the device geometry or to limit fringe current effects, the conducting electrodes are mounted to, or separated by, dielectric materials. These devices still use the standard fluid calibration procedure, but because of the precise construction, they show improved accuracy. However, they cannot be used to measure electrical conductivity of highly corrosive media since, upon contact with the dielectric, the composition of the medium is altered.